1. Field of the Invention
This invention relates to fluorescent lamp controllers and more particularly to controllers for operating fluorescent lamp or other loads at high efficiency while being very safe and highly reliable in operation and readily controllable and while achieving a long lamp life. The controllers of the invention are adaptable for use with different types and sizes of fluorescent lamps or with other loads and are readily and economically manufacturable.
2. Background of the Prior Art
It is well known that fluorescent lamps are more efficient when operated at higher frequencies. As a result of this fact and also as a result of continuing improvements in SMPS (Switch Mode Power Supply) circuits and components, there have been many proposals for using SMPS circuits operable at high frequencies for energizing and controlling fluorescent lamps. A relatively early disclosure is contained in the Wallace U.S. Pat. No. 3,611,021, issued Oct. 5, 1971. In the circuit as disclosed in the Wallace patent, a fluorescent lamp is connected in series with a capacitor to the secondary winding of a transformer which has a primary winding connected to a pair of switching transistors which alternately conduct to apply a square wave current to the oscillator the frequency of which is controlled in response to a current sense signal at the output. The secondary winding, the lamp and a capacitor are in series and form a tuned circuit which has a resonant frequency which is determined by the leakage inductance of the transformer and by the series capacitor. For starting, a second capacitor is connected in parallel with the secondary winding and the series combination of the first capacitor and the lamp, the second capacitor having a value such as to be resonant at a harmonic of the operating frequency.
The Stolz U.S. Pat. No. 4,251,752 discloses a circuit in which an inverter circuit having a constant frequency of operation is connected to a fluorescent lamp load and is supplied with a DC operating voltage developed across a capacitor by a variable duty cycle converter circuit which is connected to the output of a rectifier circuit. A loop amplifier circuit is shown having one input connected to a ramp circuit and a second input connected to the output of the rectifier circuit to be responsive to both rectifier current and voltage, the loop amplifier circuit being described as being operative to control the duty cycle of the converter circuit to maintain the input current to the rectifier in phase with the input voltage.
Additional disclosures relating to the use of SMPS circuits are contained in the Stupp et al. U.S. Pat. Nos. 4,453,109, 4,498,031, 4,585,974, 4,698,554 and 4,700,113. U.S. Pat. No. 4,453,109 discloses a high frequency oscillator-inverter with a novel leakage reactance transformer which provides not only the current limiting ballast function but also automatic control of heater power. U.S. Pat. No. 4,498,031 discloses a non-resonant coupling network which includes a reactive ballast impedance coupled between a lamp and the output of a trapezoidal waveform generator, with frequency being altered as a function of lamp current. U.S. Pat. No. 4,585,974 and U.S. Pat. No. 4,698,554 disclose driven inverters which are coupled to a lamp through non-resonant networks which include reactive ballast impedances, the frequency of the inverter as disclosed in each patent being controlled on a cycle-by-cycle basis as a function of the amplitude of lamp current. U.S. Pat. No. 4,700,113 discloses a circuit in which a high frequency inverter is coupled to a lamp through a reactive ballast impedance, the inverter being operated at a predetermined frequency until ignition occurs and its frequency being then automatically increased to a desired operating frequency.
The Zeiler U.S. Pat. No. 4,717,863 discloses a circuit which is similar to those of the Wallace and Stupp et al. patents in that frequency is controlled to control the output. Lamps are connected to the secondary winding of transformer and an inductor separate from the transformer is connected in series with a capacitor and a primary winding of the transformer to obtain an output which increases as the frequency is reduced, frequency being controlled by a photocell arrangement responsive to light output.
There are many other prior art disclosures of the use of SMPS circuits for energizing fluorescent lamp loads. Many of the prior art circuits and particularly those disclosed in the Stupp et al. patents have been very successful. However, for the most part, the SMPS circuits as proposed in the prior art have been such that they would be unduly expensive to manufacture and/or would have severe limitations with respect to performance and reliability and have not enjoyed substantial commercial success.